Many disposable absorbent articles comprise a wetness indicator composition. Wetness indicator compositions may comprise a colorant adapted to change in appearance, i.e., appear, disappear, change color, etc., upon contact with liquids such as, urine, runny bowel movements, menses, etc., in the article. Wetness indicating compositions described herein are of the substantially insoluble type such that they are designed so the composition generally remains in the same location of the article before and after being wetted with liquid. Some wetness indicator compositions are intentionally designed to simply disappear into the core of the diaper after being wetted (e.g., wetted with urine). Wetness indicator compositions of the present invention may function to serve the following: 1) the wetness indicator composition should effectively adhere, but not substantially bleed through, to the substrate on which it is applied (e.g., the backsheet) and should possess an optimum balance of cohesive strength and flexibility to remain intact during storage, as well as upon and after being wetted, 2) the initial color of the wetness indicator composition should not prematurely change color such that it confuses the caregiver or wearer as to whether a wetness event has occurred, 3) the color change of the wetness indicator composition should occur as quickly as possible after the wetness event, 4) the contrast in colors between the dry and wetted states of the wetness indicator composition should be great enough to signal the occurrence of the wetness event, 5) the color signaling the wetness event should remain visible for a long period of time after the wetness event and should not migrate to other regions of the diaper such that the signal denoting the wetness event becomes difficult or impossible to interpret, 6) the wetness indicators of the present invention should adhere, but not substantially bleed through, to the substrate that it comes in direct contact with (e.g., the nonwoven dusting layer) or that it is in close proximity to (e.g., the core cover) to provide for adequate wicking of fluid (e.g., urine), 7) the wetness indicator composition should remain stable (i.e., not change color prior to a wetness event) when placed in close proximity (if not direct contact) with higher (versus the colorant of the wetness indicator composition) pH absorbent article components, and 8) the wetness indicator composition should be easily made, easily processed for application onto the wearable article, be safe, and of economical cost.
While problems realized in the past (including high humidity and temperature environments) remain, there are additional challenges associated with new diaper designs, particularly including diaper designs comprising absorbent cores that are substantially cellulose free. These core designs comprise increased levels of absorbent polymer material, adhesives (including, thermoplastic adhesive materials), and surfactants. Each of these comprise chemical compositions that can alter the pH of the wetness indicator composition and thus cause a color change prior to a wetness event. Particularly, alkaline surfactants containing amine, amide, or quaternary functionalities are especially problematic for wetness indicator compositions comprising colorants that change color as the pH rises; but even lower pH moieties like carboxylates can also pose challenges and prematurely activate certain colorants within such wetness indicator compositions.
The color changing active used in many wetness indicator compositions are pH indicators like bromocresol green which changes color from yellow to blue in the pH range of 3.8 to 5.4. To maintain the yellow color of the bromocresol green in the dry state, the wetness indicator composition should be acidic enough to keep it in its yellow state. For a wetness indicator composition containing the bromocresol green pH indicator, it remains yellow up to the point urine contacts it and thereafter turns blue due to the pH increase. In diapers incorporating a wetness indicator composition comprising a colorant that changes color as its pH increases, it is common practice for one to incorporate acids (as well as other chemical components discussed in more detail below) within the wetness indicator composition to maintain the yellow color state of pH indicators, like bromocresol green, prior to a wetness event.
In diaper designs, placing the wetness indicator composition in close proximity to absorbent cores that are substantially cellulose free or comprising high levels of surfactants, there is a desire to optimize the acid content in relation to other materials present in the wetness indicator composition to aid in preventing its premature color change. This is especially true when the diaper design utilizes lower basis weight and more breathable substrates (e.g., backsheets, dusting layers, and core covers) because these substrates allow pH altering chemicals to come in closer proximity with the wetness indicator composition. But, while the acid content is optimized for stability, one also needs to optimize the wetness indicator composition for proper functioning of the other properties (e.g., kinetics, dye retention, stability, adhesion, etc.). If too much acid or too strong of an acid mixture is incorporated, the pH can remain suppressed even after the wetness event such that the yellow color of the bromocresol green, for instance, persists and no blue color (signaling a wetness event) results even when a wetness event has occurred. If too little acid or too weak of an acid mixture is incorporated, the wetness indicator composition can prematurely change color. Thus, the acid content should be optimized so the wetness indicator composition remains stable under various storage scenarios, as well as within new challenging absorbent article designs.
Beyond these stability issues, the wetness indicator composition should adhere to the substrate (e.g., the dusting layer) it faces that is inboard of the wetness indicator to ensure wicking of fluid (e.g., urine) to the wetness indicator composition after a wetness event. That is, when a gap exists between the wetness indicator composition and the inboard substrate (e.g., the dusting layer) that it faces, fluid may not travel from the absorbent core to the wetness indicator within the desired time to signal a wetness event. It may additionally be desired that there is substantial contact/adherence to the substrate it faces to ensure that the wetness indicator composition is evenly (top to bottom and side to side) wetted and thus provides the full signal intended. This can be accomplished by providing a wetness indicator composition that has an optimized “open time” such it is tacky enough, during its incorporation into an absorbent article, for a long enough time to adhere to the substrate, but not so long that it bleeds through the substrate.
Overall, there remains a need for a wetness indicator composition that is stable in the presence of challenging diaper designs, particularly those designs comprising: absorbent cores that are substantially cellulose free and comprising high levels of adhesive (including, thermoplastic adhesive material) and absorbent polymer material, lower basis weight dusting layers and backsheets, and dusting layers and/or core covers coated with surfactants. Particularly, there is a need for more acidic wetness indicator compositions that comprise less phosphorus and/or nitrogen (which correlates to greater colorant stability while still providing acceptable color changing kinetics and retention). Additionally, there is a need for a wetness indicator composition that has an optimized “open time” for use in absorbent articles comprising lower basis weight and breathable films and nonwovens.